All gas turbine engines include a combustor in which a mixture of fuel and air is burnt to produce exhaust gases that drive a turbine. To reduce the amount of harmful emissions such as nitrogen oxides (NOx) that are produced during combustion, most modern gas turbine engines burn a lean pre-mixture of fuel and air, without suppression of NOx by injection of water or steam into the combustion process. However, these sorts of dry low emission (DLE) gas turbine engines are particularly prone to acoustic vibrations and noise caused by variations in the gas pressure within the combustor. These pressure variations can have a frequency of 200 Hz or more, and in larger gas turbine engines the acoustic vibrations and noise can be so severe that the combustor is literally shaken to pieces.
One way of minimizing these pressure variations is to modulate the rate of delivery of the fuel flow into the combustor in a controlled manner such that the coupling mechanism which is responsible for the instability is disrupted. The present assignee has successfully modulated the fuel flow using a high bandwidth modulation valve that can operate at the necessary frequencies. The valve can be controlled to modulate a portion of the fuel flow into the combustor using a complex mathematical algorithm. However, such valves are very expensive and potentially unreliable. They also have a limited lifespan.
The purpose of the present invention is therefore to provide an alternative fluidic apparatus for modulating the rate of delivery of fuel flow into the combustor that is cheap to manufacture and very reliable.
Fluidic devices are well known to the skilled person and include bistable fluidic devices and astable (or “flip-flop”) fluidic oscillators. The general principle of operation of bistable fluidic devices and astable fluidic oscillators is explained in The Analysis and Design of Pneumatic Systems, Blaine W. Anderson, John Wiley & Sons, Inc, 1967. In bistable fluidic devices a supply jet of liquid or gas can be made to exit from either of two outlets due to the Coanda effect. The Coanda effect is the tendency of a fluid jet to attach itself to, and flow along, a wall. In bistable fluidic devices the supply jet can be made to switch from one outlet to the other by the application of a relatively small control pressure. In astable fluidic oscillators the supply jet can be made to switch from one outlet to the other continuously.
FIG. 1 shows an example of a basic bistable fluidic device 1 that includes a supply inlet 2, a pair of diverging outlets 4, 6 and a pair of oppositely facing control inlets 8, 10. The supply jet 12 has a tendency to attach itself to the side wall of one or other of the diverging outlets 4, 6. In FIG. 1, the supply jet 12 is attached to the side wall of the left-hand outlet 4. When the supply jet 12 is exiting from the left-hand outlet 4 it can be switched to the right-hand outlet 6 by the application of a control pressure to the left-hand control inlet 8. The supply jet will then continue to exit from the right-hand outlet 6 until a control pressure is applied to the right-hand control inlet 10.
An astable (or “flip-flop”) fluid oscillator can be made by connecting at least one of the diverging outlets to the control inlet on the same side. Thus, the left-hand outlet 4 can be connected to the left-hand control inlet 8, and/or the right-hand outlet 6 can be connected to the right-hand control inlet 10. The supply jet 12 can then be made to oscillate continuously so that it exits first from the left-hand outlet 4 and then from the right-hand outlet 6. The frequency of oscillation (i.e., the rate at which the supply jet oscillates between the pair of diverging outlets) depends on the length and capacity of the feedback path connecting the diverging outlets to the control inlets. Other factors that also influence the oscillation frequency include the width of the supply inlet 2, the pressure of the supply jet 12 and the angle between the pair of diverging outlets 4, 6.